Wet chemical processes are a crucial part of semiconductor device fabrication. Such processes include etching of films, removal of photoresist, and surface cleaning. Over the years, specific applications have spawned the development of numerous chemistries for wet processing, including APM (a mixture of ammonium hydroxide, hydrogen peroxide, and water), HPM (hydrochloric acid, hydrogen peroxide, and water); SPM (sulfuric acid and hydrogen peroxide), SOM (sulfuric acid and ozone), and others for specific cleaning or etching tasks. Many of these chemistries are used at or near their boiling points, since chemical reactivity, and therefore the effectiveness of the cleaning, is a function of temperature. Recent developments in wet processing technology have incorporated the use of various gases with aqueous or other liquid solutions to accomplish a desired process objective. For example, the use of ozone and water creates a strong oxidizing solution that may be useful in semiconductor processing. The use of hydrochloric acid or ammonia gas injected into water to create a low or high pH solution with specific properties are additional examples of the use of gas technology.
The use of gas/liquid process mixtures is often limited by gas solubility and temperature constraints. Solubility limitations are heightened when aqueous solutions are used. The limited solubility of gases such as ozone in water at ambient conditions, for example, limits the effectiveness of ozone/water solutions for oxidizing organic compounds, as there is simply not enough ozone available to promote the oxidation process. Reactivity constraints related to temperature are often intertwined with solubility limitations. For example, the solubility of virtually all gases in liquid solution decreases with increases in temperature. Chemical reactivity, however, increases with increasing temperature. These two factors are in conflict with each other for process optimization. Additionally, many of the aqueous solutions used in semiconductor processing are limited by their boiling points. One reason it is desirable to avoid boiling is to prevent cavitation and suppress bubble formation for more effective use of megasonic waves in cleaning wafer surfaces. For example, a 5:1:1 mixture of water, ammonium hydroxide, and hydrogen peroxide will boil at approximately 65 C. Accordingly, such a mixture cannot be maintained in liquid form at elevated temperature unless the composition is changed to elevate the boiling point.
A critical step in the wet-processing of semiconductor device wafers is the drying of the wafers. Any rinsing fluid that remains on the surface of a semiconductor wafer has at least some potential for depositing residue or contaminants that may interfere with subsequent operations or cause defects in the end product electronic device. In practice, deionized ("DI") water is most frequently used as the rinsing fluid. Like most other liquids, DI water will "cling" to wafer surfaces in sheets or droplets due to surface tension following rinsing. An ideal drying process would operate quickly to effect the removal of these sheets or droplets and leave absolutely no contaminants on the wafer surfaces, while presenting no environmental or safety risks.
Although various technologies have been used to dry wafers and reduce the level of contaminants left on the wafer surface after drying, the most attractive technology currently available falls under the broad category of surface tension trying. A typical surface tension dryers accomplishes wafer drying using the following steps: (1) wafers are immersed in a rinse medium; (2) the rinse medium is either drained away from the wafers or the wafers are lifted out of the rinse medium, exposing them to a displacement medium that is typically an inert carrier gas containing a percentage of organic vapor, usually an alcohol, such as isopropyl alcohol ("IPA"); (3) the organic vapor dissolves in the surface film of the rinse medium, creating a concentration gradient in the liquid, which in turn creates a surface tension gradient that enables the higher surface tension in the bulk liquid to essentially "pull" the lower surface tension liquid away from the wafer surface along with any entrained contaminants to yield a dry wafer; and, in some instances, (4) the displacement medium may be purged from the locale of the wafer using a drying medium such as an inert gas stream. Additionally, the carrier gas may be heated to assist in drying and to prevent liquid condensate from forming on the wafer surfaces.
Conventional surface tension drying technology is limited by at least the following factors: (1) it involves the inherent hazard of causing IPA, a flammable liquid, to be boiled at a temperature well in excess of its flash point; (2) it requires the consumption of IPA at relatively high rate; and (3) it creates relatively high fugitive organic vapor emissions.
In light of the limitations inherent to these and other processing and drying technologies, it is an object of one aspect of the present invention to suppress the boiling point of a wafer processing liquid to permit processing at elevated temperatures.
It is an object of another aspect of the present invention to increase the solubility of gases in the liquid phase to enhance chemical reactivity.
It is yet another object of the present invention to prevent cavitation and suppress bubble formation for more effective use of megasonic waves to enhance cleaning performance.
It is still another object of the present invention to reduce or eliminate the need for using an organic vapor as a drying or displacement medium in a wafer drying process.
The term "wafer" means a semiconductor wafer, or similar flat media such as photomasks, optical, glass, and magnetic disks, flat panels, etc.